Succession rule

[Jolt]

I've tried the matrilinieal option (I think it's called), where whoever marries the FL's first daughter becomes the FH, and their first daughter determines the next heir etc.

The closest thing there was (AFAIK there was no such rule as "Only males can inherit but female primogeniture isn't discriminated in any way") is the Semi-Salic Primogeniture, which descendants of a female can inherit only if all her brothers, and their children are dead. Or you have the Abolute Cognatic Primogeniture, which the elder child inherits everything regardless of gender.

EDIT: Depends on the Faction. For a Monarchy, I select the Elder-most FM of the FL. For a Republic, I go with Influence. For a Barbarian Confederation, I choose the "Strongest" (Most Command Stars).